I have black leather seats and I do find them to be hot (my husband does not). I bought a seat cover for the driver's seat and that helps (but I'm sure you wouldn't want to do that for a new car). My daughter is 18mo and seems to get - I'm not sure what, half the time! - "stuff" in the back seat that shows up very easily - that didn't seem to show up on the gray seats that I had before. Black just seems to show any and all specks of - you name it: dirt, leaves, crumbs of any kind, lint, bits of paper scraps that fall from new books (my daughter loves to read in the car). Plus, our dash is black and I keep a microfiber rag in the glove box to go over the dash frequently as it shows the dust very easily as well.
If your children are older, the dirt/lint/stuff on the seats might now be a problem and different kinds of leather feel differently, so the hot problem might not exist in the car(s) you are looking at.
Sunshades help a lot with the hot seats.
P.